During drilling operations for the extraction of hydrocarbons, a variety of recording and transmission techniques are used to provide or record real-time data from the vicinity of a drill bit. Measurements of the surrounding subterranean formations may be made throughout drilling operations using downhole measurement and logging tools, such as measurement-while-drilling (MWD) and/or logging-while-drilling (LWD) tools, which help characterize the formations and aide in making operational decisions. Wellbore logging tools make measurements that may be used to determine the electrical resistivity (or its inverse conductivity) of the formations being penetrated, where the electrical resistivity indicates various features of the formations. Those measurements may be taken using one or more antennas coupled to the wellbore logging tools.
Logging tool antennas are often formed by positioning coil windings about an axial section of the logging tool, such as a drill collar. Ferrite materials or “ferrites” (or magnetic materials in general) are sometimes positioned beneath the coil windings to increase the efficiency and/or sensitivity of the antenna. The ferrites facilitate a higher magnetic permeability path (i.e., a flux conduit) for the magnetic field generated by the coil windings, and help shield the coil windings from the drill collar and associated losses (e.g., eddy currents generated on the drill collar). Since the ferrites have the capability of shaping and manipulating the generated magnetic field, great care must be taken when designing the ferrite geometry to prevent undesired effects. Due to the increased use of tilted or directional coil windings, however, the shape of the ferrites has become quite complex, and very difficult and/or cumbersome to machine.
As a result, ferrites are more recently being molded and/or pressed into desired geometric shapes and configurations. Briefly, molding ferrites includes mixing a powder iron/ferrite material with a binder, such as a silicone-based rubber, an elastomer, an RTV, a polymer (such as polyimide), a ceramic, or an epoxy. The mixture is then pressed into a mold, where it conforms to the intricate geometry of the mold. This molding process, however, can result in non-homogenous ferrites or ferrites that exhibit a large variance in relative permeability (i.e., non-homogeneity) even when molded from the same mixture. Such non-homogeneity and variance in permeability may adversely affect field roundness and/or symmetry, which is critical for logging tools.